The high-resolution detection of gamma-rays is useful in applications such as the detection of nuclear material or material that has been exposed to high energy radiation. Emissions from radioactive materials such as uranium or plutonium provide unique signatures that, if accurately measured, can indicate the age and enrichment of the material and sometimes its intended purpose or origin. Currently, the most promising methods of detecting gamma-rays use semiconductors having high atomic numbers and wide band-gaps such as HgI2, CdTe and CdZnTe.
Semiconductors having high atomic numbers generally have high detection efficiency, do not require cryogenic cooling, and have the potential for very high resolution. Currently, the preferred use of these semiconductors is in a co-planar grids (CPG) configuration. P. N. Luke, “Electrode configuration and energy resolution in gamma-ray detectors” Nuclear Instruments and Methods in Physics Research A 380 (1996) 232-237, herein fully incorporated by reference, describes the co-planar grids (CPG) configuration. This concept is illustrated in the CPG 1 shown in FIG. 1.
The typical CPG, shown in FIG. 1, generally comprises a semiconductor substrate 3 having a cathode electrode 5 on a first side and an adjacent second side having a collecting grid 7 and a non-collecting grid 9. The collecting grid 7 and the non-collecting grid 9 are coplanar and interdigitally positioned covering the second side of the substrate 3. The collecting grid 7 and non-collecting grid 9 are interdigitally positioned whereby the collecting grid 7 and the non-collecting grid 9 both at least partially cover the second side of the substrate 3 in an alternating fashion. For example, the collecting grid 7 and non-collecting grid 9 commonly cover the second side of the substrate 3 using a pattern whereby the collecting grid 7 covers about 500 microns followed by the non-collecting grid 9 covering the next 500 microns. A small bias voltage is applied across the collecting grid 7 and the non-collecting grid 9 so that electrons in proximity of the grids are collected by the collecting grid 7. A highly negative voltage is applied to the cathode electrode 5, thus generating an electric field in the substrate 3 moving electrons towards the collecting grid 7 and non-collecting grid 9.
Ionizing radiation generates charges (electrons and holes) within the substrate 3. The electrons move in the substrate 3 towards the collecting grid 7 and non-collecting grid 9, inducing an identical voltage on the collecting grid 7 and the non-collecting grid 9. The same occurs for the much slower holes, moving towards the cathode 5. Electrons approaching the grids are collected by the collecting grid 7, inducing a measurable voltage on the collecting grid 7. Therefore, a voltage difference between the collecting grid 7 and the non-collecting grid 9 occurs only when the electrons are collected by the collecting grid 7. Since, electrons within the substrate 3 induce an equal voltage to both the collecting grid 7 and non-collecting grid 9 they have no affect on the voltage difference between the collecting grid 7 and the non-collecting grid 9. Therefore, ionizing events may be detected using the voltage difference between the collecting grid 7 and the non-collecting grid 9.
The collecting grid 7, shown in FIG. 1, is typically connected by a wire 101 to a collecting grid amplifier 11, which produces a low voltage signal representative of the voltage induced on the collecting grid 7 by ionizing events. Likewise, the non-collecting grid 9 is typically connected by a wire 103 to a non-collecting grid amplifier 13, which produces a low voltage signal representative of the voltage induced on the non-collecting grid 9 by ionizing events. Both the collecting grid amplifier 11 and the non-collecting grid amplifier 13 are each connected by wires (105 and 107) to a difference amplifier 15, which is capable of determining the difference of the collecting grid amplifier 11 and the non-collecting grid amplifier 13 signals. Essentially the output of the difference amplifier 15 is the difference in the voltages of the collecting grid 7 and the non-collecting grid 9. This difference is due to the collection at the collecting grid 7 of electrons freed by ionizing radiation.
By making the detector insensitive to the electrons and holes traveling in the substrate 3 the resolving capability of the system is greatly enhanced. On the other hand, the system is still limited by the trapping effect, energy loss due to electron trapping in the substrate 3. The longer the electrons travel in the substrate 3 the more energy they lose. Consequently, the charge associated with an ionizing event still shows a residual dependence on the depth of the ionizing event that can substantially limit the resolution of the detector.
In order to compensate for the trapping effect three techniques have been proposed (a) lowering the gain of the non-collecting grid 9 relative to that of the collecting grid 7, (b) weighting each event by measuring its depth of ionizing event through measurement of the amplitude of the cathode 5 voltage, (c) using timing information from the collecting grid 7 and non-collecting grid 9 to calculate depth information and weighing each event by its calculated depth.
The relative gain compensation technique was proposed by Paul Luke in 1996 and is shown in FIG. 2. The relative gain CPG 17 is essentially the CPG 1 of FIG. 1 further comprising a gain amplifier 19 positioned between and connected by wires (109 and 111) to the non-collecting grid amplifier 13 and the difference amplifier 15. The gain amplifier 19 reduces the gain of the non-collecting grid amplifier 13 voltage relative to the gain of the collecting grid amplifier 11 voltage. With this approach a small amount of induction from electrons traveling in the substrate 3 is reintroduced into the system. The amptitude of the deep-ionizing events (ionizing events close to cathode) is increased, while the non-deep-ionizing events (ionizing events close to grids) is reduced, resulting in a first order compensation of the trapping effects. The optimum value of the relative gain typically ranges between about 0.6 and 0.9, depending on the quality of the sensor, the voltage difference between the collecting grid 7 and the non-collecting grid 9, and the temperature of the sensor. Generally, instead of a gain amplifier 19, shown for clarity, the gain is typically adjusted by modifying the value of a passive element in the difference amplifier 15, typically a resistor.
Unfortunately, the correction by the relative gain compensation technique is roughly linear and provides only a first order compensation for the trapping effects, resulting in over compensation for some ionizing events, and under compensation for others. Therefore, this technique has a limited resolution. Furthermore, if the relative gain needs to be modified (due to replacement of the sensor or a change in voltage and/or temperature), a hardware change is needed, which is impractical especially in commercial applications.
The cathode technique was proposed in Z. He, G. F. Knoll, D. K. Wehe, R. Rojeski, C. H. Mastrangelo, M. Hamming, C. Berret, and A. Uritani, “1-d position-sensitive single carrier semiconductor detectors”, Nuclear Instruments and Methods in Physics Research A 380 (1996), 228-231, hereby fully incorporated by reference. FIG. 3 shows a typical CPG using the cathode technique 25, essentially the CPG 1 of FIG. 1 further comprising a cathode amplifier 27. The cathode amplifier 27 is connected by a wire 113 to the cathode electrode 5. By calculating the ratio between the voltage of the cathode amplifier 27 and the voltage of the difference amplifier 15, it is possible to extract the depth of an ionizing event. The difference amplifier 15 voltage of an ionizing event can be corrected (weighted) for the trapping effect according to the value of an associated depth of the ionizing event, gathered by calibrating the sensor. The optimum weighting parameters depends on factors such as the quality of the sensor, the voltage difference between the collecting grid 7 and the non-collecting grid 9, and the temperature of the CPG 25.
The cathode technique allows a higher order compensation of the trapping effects, thus potentially achieving a better resolution when compared to the relative gain technique. However, more complex electronics are needed for the additional cathode amplifier 27. These electronics are more complicated than the collecting amplifier 7 and the non-collecting amplifier 9 since the cathode amplifier 27 must operate at extreme negative voltages in the order of thousands of Volts, compared to the typical 0-60 Volts operating voltage of the collecting amplifier 7 and non-collecting amplifier 9. For example, the cathode amplifier 27 typically requires a bulky high voltage capacitor to couple the high voltage cathode to a charge amplifier.
Another more recent method developed is the timing of grids technique proposed by G. De Geronimo et al. in U.S. patent application Ser. No. 11/174,241. This technique uses the CPG of FIG. 1 and consists of measuring the depth of ionizing event by using the timing information from the collecting grid amplifier 11 and the difference amplifier 15. The difference signal of the event is corrected (weighted) for the trapping effect according to the value of an associated depth of ionizing event.
The timing difference between two signals is a measure of the depth of ionizing event. The first timing signal occurs in correspondence of the ionizing event, detected by the collecting grid amplifier 11. The second timing signal occurs when the charge approaches the collecting grid 7, detected by the difference amplifier 15. In case of the deepinteracting events (ionizing events close to cathode) the associated timing difference is large, while in the case of the non-deep-interacting events (ionizing events close to grids) the timing difference is small.
Unfortunately, the timing of grids technique requires complex timing signals and related logic circuitry. The timing signals and logic also complicates noise considerations of the sensor design from the interaction between the digital logic circuitry and the analog components utilized in this method.
Therefore, there is a need for a method of accurately detecting ionizing events in real time.